In recent years, utilizations of a higher dielectric constant thin film called high-k film for the application of a material composing of a semiconductor device are investigated. Typical high-k materials include oxides containing Zr, Hf or the like. Novel superior device performances can be achieved by employing such materials for a gate insulating film of MOSFET and a capacity film of a capacitance element.
JP-A-2003-8004 (paragraphs [0026] to [0029] and FIG. 1) describes a MOSFET having a gate insulating film of a multi-layered structure (Al2O3—HfO2— Al2O3) comprising aluminum oxide films formed on both of an upper and a bottom surfaces of a hafnium oxide film. When the high-k material is employed for a gate insulating film of a transistor, thinner film thickness converted into the silicon oxide film may be applicable even if the thickness of the gate insulating film is designed to be thicker in a certain level, thereby providing the gate insulating film which is physically and structurally stable.
In the preparations for forming the transistor comprising such gate insulating film, it is necessary to remove gate insulating films formed in regions where gate electrode is not formed after the gate electrode is fabricated. If this gate insulating film is not removed to remain thereon, the undesired short-channel effect may be remarkable, thereby reducing the reliability of the transistor.
Nevertheless, the hafnium oxide film is generally difficult to be etched. In the above-described patent application, it is described that the gate insulating film can be etched off by conducting reactive ion etching (RIE), and it is also described therein that if it is not sufficient with RIE, plasma etching may be employed. However, in reality, it is not easy to remove hafnium oxide film by conducting the dry etching. In particular, it is necessary to conduct a thermal processing at relatively higher temperature in the step for annealing the gate electrode or the like during the manufacturing process for the MOSFET. During this step, crystallization of the hafnium oxide film is caused to further convert the hafnium oxide film into a film that is more difficult to be etched.
In addition, when the hafnium oxide film is dry etched, there might be a problem of plasma damage to an underlying material of the high-k material. In addition, silicon substrate is undesirably etched by the dry etching of the hafnium oxide film to vary the junction depth of the impurity diffusion layer of the transistor, thereby increasing the leakage current therefrom.
On the other hand, even if the hafnium oxide film is to be removed via the wet etching, the removal thereof may not easily be carried out. Such circumstances are described in No. 50 Extended Abstracts, Japan Society of Applied Physics and Related Societies (Oyo Butsurigaku Kankei Rengo Koenkai Koen Yokoshu), No. 2 (issued Mar. 27, 2003, at Kanagawa University), p.p. 934 (29a-ZW-5), entitled “Wet etching of HfO2 by irradiating ultra-violet ray (Shigaisen-o shosha suru HfO2 no wet etching)”, and it is described in the literature that the hafnium oxide film is difficult to be etched off and it is also described that the etching thereof becomes possible if the wet etching of such film with phosphoric acid under the exposure to the UV light is conducted. Conversely, there is nothing more difficult film for being etched off than the hafnium oxide film so that such special processing must be conducted.
In addition, it is critical that the surface of the silicon substrate as an underlying material should not be damaged when the etching is conducted in the case of carrying out the wet etching. As described earlier, it is general to form a silicon thermal oxide film between the film consisting of high-k material and the substrate, and an etchant available for etching the hafnium oxide film may also ordinarily etch the silicon thermal oxide film, so that these films are simultaneously removed with the identical etchant, and eventually the surface of the silicon substrate is exposed. When such process is adopted, silicon substrate is readily damaged. Such problem is similarly occurred when silicon native oxide film is chemically formed during a process step for cleaning the substrate employing a sulfuric acid-hydrogen peroxide mixture (SPM) and/or an ammonia-hydrogen peroxide mixture (APM), in addition to the case of forming the silicon thermal oxide film.
Further, in the case of wet etching, a device isolating film of shallow trench isolation (STI) structure is exposed to the surface by the etching, and thus problems of causing dissolution and/or damage of the exposed device isolating film may be caused when the above-described etchant is used. This is because the device isolation film is ordinarily constituted of a silicon oxide film, which is readily dissolved by using the etchant available for etching the hafnium oxide film.